Outboard motors equipped with an exhaust release pipe (auxiliary exhaust pipe) of the type described are known as disclosed, for example, in Japanese Patent Laid-open Publication (JP-A) No. 9-039890.
The disclosed outboard motor has an engine room defined by an engine cover, and an exhaust expansion chamber defined by a driveshaft housing disposed below the engine cover. The engine room and the exhaust expansion chamber are separated or isolated from one another by an engine holder on which an engine is supported with an oil-seal housing disposed between the engine and the engine holder. A duct assembly having an inverted U-shaped configuration is mounted on the engine holder and extends vertically across the engine holder so that one end of the duct assembly opens to the exhaust expansion chamber for allowing entry of the engine exhaust gas and the other end of the duct assembly extends deeper from the exhaust expansion chamber for allowing entry of the water. The U-shaped duct assembly defines an exhaust discharge passage. The engine holder has an exhaust release passage formed therein. The exhaust release passage is connected at one end to the exhaust discharge passage and opens at the other end to the atmosphere outside the outboard motor.
During operation of the outboard motor, when the discharge pressure of the engine is high, exhaust gas discharged from the engine is converged into the exhaust expansion chamber, flows upward and then downward along the exhaust discharge passage defined by the inverted U-shaped duct assembly, and is finally discharged into the water. When the engine stops running or when the engine is idling, the exhaust pressure of the engine is low and the engine exhaust gas cannot be discharged into the water. Under such condition, the exhaust gas is released from exhaust expansion chamber to the atmosphere outside the outboard motor through the exhaust release passage.
In the disclosed outboard motor, the engine holder has a tubular portion forming an outlet part of the exhaust release passage and projecting outward from a circular opening formed in a rear end wall of the engine cover. A ring-shaped elastic seal member or packing is fitted in the circular opening of the engine cover so as to form a hermetic seal between the tubular engine holder portion and the engine cover.
The conventional outboard motor of the foregoing construction has a problem that due to cumulative tolerances involved in the manufacture and assembly of the engine cover and the engine holder, the tubular portion of the engine holder and the circular opening of the engine cover tend to become off-centered or eccentric with each other. Due to the eccentricity or offset between the tubular engine holder portion and the engine cover's circular opening, the elastic ring-shaped packing is distorted into a partly compressed and partly stretched configuration (that is, the ring-shaped packing is radially compressed at one side and radially stretched at another side diametrically opposite to the one side). The packing thus distorted cannot provide a uniform contact pressure between itself and the engine cover and also between itself and the tubular engine holder portion, failing to form a reliable seal between the tubular engine holder portion and the engine cover. Especially when the offset between the tubular engine holder portion and the engine cover's circular opening is large, the packing may be compressed and stretched beyond its elasticity. In such instance, a compressed side of the packing may cause fatigue, and a stretched side of the packing may separate from the tubular engine holder portion or the engine cover, allowing entry of the water into the outboard motor.
Attempts to reduce the offset between the tubular engine holder portion and the engine cover's circular opening may require precision working, which is time-consuming, lowers the assembling efficiency and increase the cost of the outboard motor.